In order to achieve lamp dimming in a pre-existing fluorescent lighting system, it is desirable to limit the required modifications only to the distribution panel. Since the lamp ballast is nearly always inaccessibly mounted in or near the lamp fixture, a retrofit fluorescent dimming system should accomplish dimming using a conventional ballast. It is known in the art to cause dimming of a fluorescent lamp by manipulating the 60 hertz line voltage supplied to the conventional ballast.
One method for manipulating the voltage supplied to the ballast is to notch the input waveform, as shown, for example, by U.S. Pat. No. 4,350,935 to Spira et al. A non-conductive region is formed in the input waveform by opening a series switch for a non-zero portion of each half wave of the input waveform. These periods of zero energy transfer result in a decrease in the RMS voltage applied to the ballast and, therefore, to the lamp, resulting in dimmed light output.
By reducing the RMS voltage applied to the ballast, the previously described notching scheme has the disadvantages that lamp filament power decreases and that cathode drop increases due to the loss of filament heating. As cathode drop increases, fewer electrons are emitted to initiate the plasma building process. Thereafter, electrons tend to be torn from the lamp filaments by field emission, blowing off pieces of the emission mix and leading to hard lamp starting, reduced lamp life, and excessive blackening of the lamp ends. In the circuitry of the aforementioned Spira et al. patent, special precautions must be taken to ensure that the lamps are not started in a dimmed condition. Thus, in Spira et al., the input waveform is not notched until the lamp has reached full operating temperature under full line voltage.